There is a demand for a main line optical communication system having a high capacity, because an internet traffic is increased. An optical signal to noise ratio (OSNR) tolerance is degraded when a bit rate per one wavelength is enlarged. In this case, a signal quality may be degraded more because of chromatic dispersion of a transmission path, polarization mode dispersion, waveform distortion caused by non-linear effect or the like. Therefore, a digital coherent receiving method has attracted attention because the method is expected to improve the OSNR tolerance and waveform distortion tolerance of a transmission path. D. Ly-Gagnon, IEEE JLT, vol. 24, pp. 12-21, 2006 discloses the method.
The digital coherent receiving method is a method where optical intensity and phase information are extracted with a coherent receiving method, and the extracted optical intensity and the extracted phase information are digitalized by analog-to-digital convertor (ADC), and the received signal is demodulated in a digital signal processing circuit by digitalized signal. The digital coherent receiving method has favorable characteristics with respect to a high-bit-rate optical transmission, because the digital coherent receiving method improves the OSNR tolerance with the coherent receiving method and compensates for waveform distortion with the digital signal processing circuit.
However, Darko Zibar et al, ECOC 2009, 7. 3. 4 discloses that the digital coherent receiving method has a problem that a sampling phase shift detector circuit has a low tolerance to waveform distortion caused by chromatic dispersion, polarization mode dispersion or the like, although the digital coherent receiving method has high performance with respect to waveform distortion compensation with a digital signal processing. Especially, P. M. Krummrich et. al, OFC 2004, FI3 discloses that the polarization mode dispersion of a transmission of a path fluctuates speedily because of fluctuation of polarization condition of the transmission path. There is a demand for a sampling phase shift detecting method that may tolerate fluctuation of polarization mode dispersion in operation and has high tolerance with respect to the waveform distortion.